behaviour of a confined fire located in an unventilated zone

Proceedings

Proceedings of the Combustion Institute 31 (2007) 2529–2536

www.elsevier.com/locate/proci

of the

CombustionInstitute

Behaviour of a confined fire locatedin an unventilated zone

Anthony Pearson a, Jean-Michel Most a,*, Dougal Drysdale b

a CNRS, University of Poitiers, ENSMA, Laboratoire de Combustion et de Detonique, UPR 9028,

1 avenue Clement Ader, BP 40101, Teleport 2, 86961 Futuroscope Chasseneuil Cedex, Franceb Institute for Infrastructure and Environment, The University of Edinburgh, Rankine Building,

The King’s Building, Edinburgh EH9 3JN, UK

Abstract

The behaviour of a fire in an enclosure is studied for a configuration where the fuel source is located inthe upper hot unventilated zone trapped by a soffit. The experimental study, undertaken in a laboratory-scale compartment with a fuel source above the level of a soffit, included the determination of theparameters (ventilation factor, rate of fuel supply) controlling the combustion or leading to extinction.Measurements (PIV, thermocouples, gas sampling and analysis) were performed to propose a hypothesison the structure of the flame (flame stabilisation mechanisms, premixed or diffusion types). Video photog-raphy is used to determine the area covered by the flames. This information is used as a criterion to identifythe combustion regimes. The results show that the gaseous fuel is diluted in the combustion products (CP)in the upper layer and that a recirculatory motion is formed, driven by buoyancy forces, which enhancesthe mixing of fuel and CP. These then travel horizontally towards the vent along the interface between thelower fresh air and upper zones, and are premixed with the convected air in the enclosure, before enteringthe reaction zone and being burnt. The flame stabilises at the interface between the upper hot and lowerventilated layers in the compartment. The observed ‘‘ghosting flame’’ is stabilised by a triple flame if theflame speed of the premixed flame is higher than the natural convection velocity induced in the compart-ment. The flame stability is quantified by a criterion based on the area of the horizontal flame. It has beenobserved that the combustion is controlled by the available mass fuel flux at the reaction zone if theventilation is sufficient. This information is essential for the modelling of the phenomena involved in fireswith such an underventilated fuel source.� 2006 Published by Elsevier Inc. on behalf of The Combustion Institute.

Keywords: Compartment fires; Ghosting flame; Triple flame; Flame stabilisation mechanisms; Ventilation factor

1. Introduction

Over the past decades, much work has beendone to increase the understanding of compart-

1540-7489/$ - see front matter � 2006 Published by Elsevier Idoi:10.1016/j.proci.2006.08.019

* Corresponding author. Fax: +33 0 549498291.E-mail address: [email protected]

(J.-M. Most).

ment fires [1–3]. However, the current knowledgeof the structure of the flames and the flow fieldsencountered in compartment fires is still insuffi-cient to improve the combustion models appropri-ate for the numerical prediction of the growth ofcompartment fires.

Most previous studies have concentrated onscenarios with the fuel source on or near the floor,with air being entrained at the base of a diffusion

nc. on behalf of The Combustion Institute.

mailto:[email protected]

2530 A. Pearson et al. / Proceedings of the Combustion Institute 31 (2007) 2529–2536

flame anchored on the edge of the fuel source.Less well explored is the case where the fuel sourceis near the ceiling. If the enclosure has a soffitabove the vent, the hot combustion product gases(CP) are trapped under the ceiling and may envel-op the fuel source, preventing air from reaching it.The flame decreases in efficiency as the air is con-sumed. Unburnt fuel enters the upper hot zonewhere it is diluted (the fuel vapour continues tobe pyrolysed). The combustion is either extin-guished or leaves the surface of the fuel sourceto be stabilised at the interface between the hotupper and fresh air zones of the compartment.

This scenario was partly inspired by a potentialfire scenario with ruptured pipes or overheatingcables, lights or accessories near the ceiling actingas a fuel supply located, after an equilibrium time,in the upper vitiated layer in a compartment.

In a laboratory-scale study of such fire condi-tions, Coutin et al. [4–6] described eight differentcombustion regimes, distinguished by the shape,size and colour of the flames and depending onthe fuel flow rate, the geometry, the fuel sourcelocation and the ventilation factor. Among themis the stabilisation of the combustion at the inter-face between the hot and cold zones. This regimebears resemblance to the blue flames observed byMorehart et al. [7,8] during experiments per-formed with a gas burner placed below a hoodto study the composition of the upper layer duringthe transient development of a fire. The flamedetached itself from the burner and hovered ‘‘ata distance of 10–15 cm above the burner surface’’.

Coutin’s flame is also reminiscent of the‘‘ghosting flames’’ observed by Sugawa andKawagoe [9] during tests with pool fires in a com-partment with extremely limited ventilation. Theyreported that after an initial phase when anorange diffusion flame burnt above the pool,‘‘the flame detached. . .from the fuel surface’’ and‘‘floated just under the ceiling’’. It ‘‘looked like athin film as an aurora’’.

Similar flames have also been described inother conditions, such as the wandering flamesdescribed by Audouin et al. [10] from full-scaletests (convection of a reacting zone structuredetached from the pool fire through a room upto the aperture). The observed phenomena canalso be related to re-ignition of fuel pockets inhot zones with non-homogeneous in oxidizerconcentrations.

Though these flames were observed in real con-ditions, more work is needed to enable the develop-ment of models of poorly ventilated compartmentfires, associated particularly with backdraft.

These behaviour patterns also show similaritiesto those observed during so-called ‘‘flamelesscombustion’’ in industrial regenerative furnaces[11] where the reactants are mixed with combus-tion products at a higher temperature prior tobeing fed into the flame.

2. Aims and concept

The present study aims at characterising theaerodynamics, thermodynamics and chemistry ofthe flames which occur when a fuel source is locat-ed inside the smoke layer. The present study con-centrates on regimes of flames stabilised along thelayer interface: the gases are diluted in combus-tion products. There is still no comprehensivehypothesis available to describe the mechanismby which the combustion is stabilised.

The objectives are to provide hypotheses onthe mechanisms controlling the combustion inorder to develop physical models.

In particular, the following work wasperformed:

• the description of the flame structure,• the demarcation between the different combus-

tion regimes,• the determination of the aerothermochemical

mechanisms controlling the combustionregime,

To obtain a quasi steady-state configurationand to control the input power of the fuel source,the pyrolysis of a solid fuel was simulated byinjecting a fuel gas through a burner: consequent-ly the fuel flow rate (thermal input power) couldbe kept constant and was not correlated with theheat flux to the fuel source.

The air supply was provided by natural con-vection induced by the heat released from thecombustion.

Coutin qualitatively concluded that the blueinterface flame is probably stabilised by a tripleflame mechanism, i.e. it comprises of premixedflame burning in a partial fuel-oxidant mixtureand a trailing diffusion flame in which combustionof leftover unreacted species occurs [12–14]. Thiswork aims to confirm this hypothesis.

3. Experimental apparatus

3.1. Compartment

The volume of the compartment is 0.21 m3

(0.62 m long · 0.85 m high · 0.40 m wide) wherethe front is open except for a soffit above the ventof 0.20 or 0.35 m depth, Hsoffit (Fig. 1) [4–6]. Theupper half of the compartment is insulated withmineral fibreboard. Windows of ceramic glassare incorporated in the walls and the floor toallow the observation of the flame and the useof optical diagnostics. A channel with a rectangu-lar section open towards the bottom is attached tothe compartment exit at the level of the soffit. Thecombustion products and unburnt gases flow andburn in this channel, thus allowing the evaluationof the amount of fuel burning inside and outside

0.62 m

HVent

HSoffit

0.40 m

0.85 m

thermocouple tree

channel

vent

soffit

burner

Fig. 1. Schematic of the experimental apparatus.

A. Pearson et al. / Proceedings of the Combustion Institute 31 (2007) 2529–2536 2531

of the compartment: the air entrainment into thereaction zone is from below for both the internaland external part of the flames, and is thereforeassumed to be similar for both the internal andexternal parts of the flame.

3.2. The burner

The burner consists of a line source with eigh-teen orifices of 2 mm diameter to supply the fuel(propane) and is positioned horizontally and par-allel to the compartment opening. This configura-tion produces a nearly two-dimensional flow.

3.3. Experimental parameters

The parameters of this experimental study arethe geometrical characteristics of the compart-ment (size and shape of the enclosure, soffit andopening), the position of the burner in the com-partment, the fuel composition and its mass flowrate _m. To characterise a fire, the literature gener-ally uses the input thermal power _Q; in this work,a further parameter is defined: the mass flux offuel _m00 in the trapped upper zone of thecompartment,

_m00 ¼ _mAc

in kg=m2=s

where Ac is the surface of the floor of thecompartment.

4. Diagnostics

4.1. Visualisation of spontaneous emission fromflames

The light emitted spontaneously by the flamewas recorded with digital video photography from

the side and from underneath. These allow thesize, shape and location of the flames to be record-ed. A relation is sought between the heat release _Qand the area of the horizontal flame.

A statistical processing on a minimum of 50images per case allowed the determination of themean surface areas covered by the flame. Histo-grams of the grey scale were calculated to deter-mine a clear threshold between the ‘‘almostblack’’ of the background and the grey of theflames. A sensitivity study showed that for sam-ples of more than 50 images, the obtained averageflame size so obtained did not alter by more than3%.

4.2. Particle imaging velocimetry (PIV)

Particle imaging velocimetry measurementswere performed to determine the gas velocity field,particularly near and behind the edge of the flamestructure at the interface.

The PIV equipment included a twin cavity125 mJ pulsed Nd:YAG laser, a 1240 · 1048 pixelCCD camera and image processing software. Thefield of view was roughly 170 · 140 mm2. Theacquisition frequency was 2 Hz, which does notallow a temporal resolution of the flow or themovement of the convective flame.

Seeding was done with zirconium oxide parti-cles injected with a flow of nitrogen into the upperlayer. It was verified that this option qualitativelyminimised the aerodynamic and chemical distur-bance of the flows and combustion.

To couple the position of the flame with thevelocity field in order to determine the flow veloc-ity just upstream of the flame structure, a longerexposure time was set for the 2nd PIV image ofeach pair, thus allowing the light from the flameto be captured together with the laser light scat-tered by the seeding particles. A liquid crystal

bright blue edge

rear front

flame sheet

Fig. 2. Photograph of a blue flame taken from below;Hsoffit = 0.35 m, _m00 ¼ 1:2� 10�3kg=m2=s. (For interpre-tation of the references to color in this figure legend, thereader is referred to the web version of this paper.)


shutter manages this operation. The appropriateexposure time depends on the luminosity of theflames—about 15 ms for bright yellow flames,50 ms for blue flames. The bandpass filter normal-ly used to block out light other than that from thelaser needs to be removed from the camera; thetests are performed in darkness.

A mean flow velocity can be obtained in acoordinate system linked by coordinate transfor-mation relative to the flame edge.

4.3. Temperature

Temperature profiles in the median plane weredetermined with a vertical thermocouple tree. Sev-en type K thermocouples of 50 lm diameter wereplaced at heights of 0.15, 0.23, 0.35, 0.45, 0.47,0.59 and 0.69 m. The data presented here wereproduced with the tree located 0.1 m from the rearwall and 0.14 m from the right-hand wall. Due tothe relatively low difference in temperaturebetween the walls and the fire, the effects of radi-ation on the thermocouple signal were neglected.

4.4. Chemical species

An isokinetic probe took samples of gas. Themean stable chemical species were analysed bygas chromatography (hydrocarbon compounds)or with online analysers for hydrocarbons, O2,CO and CO2. Due to the motion of the reactionzone and of the large time constant of the equip-ment (c. 1 min), only mean species concentrationsare obtainable.

4.5. Operating conditions

The tests were performed using propane withtheoretical thermal outputs _Q ranging from 8 to33 kW, corresponding to a mass flow rate varyingfrom 0.17 to 0.71 g/s, or mass fluxes of0.69 · 10�3 kg/m2/s to 2.9 · 10�3 kg/m2/s, cover-ing the whole identified domain. To obtain theselected combustion regime (ghosting flame withfloating blue flame), the propane flow rate mustbe within the range of 0.20 g/s to 0.33 g/s_m00 ¼ 0:81� 10�3kg=m2=s to 1.3 · 10�3 kg/m2/swith the burner placed just underneath the ceiling,well above the level of the soffit.

5. Characterisation of the flame structure

5.1. Visualisation of the light emitted by the flames

When the deep soffit (Hsoffit = 0.35 m) isinstalled, a minimal fuel flow rate of 0.20 g/s—equating to a fuel mass flux _m00 ¼ 0:81 �10�3kg=m2=s—is required to sustain combustion.

For _m00 in the range of 0.81 · 10�3 kg/m2/s and1.3 · 10�3 kg/m2/s, the flame is stabilised horizon-

tally and presents itself as a bright blue edge(Fig. 2) and a blue sheet which is much less brightand is inclined upwards towards the vent at anangle of between 15 and 20�, covering the wholeregion between the edge and the vent. The colouris attributed to the spontaneous emission fromhydrocarbon radicals CxH

�y —the conditions are

not favourable for the production of soot. Asalready remarked by Morehart et al. [7,8], thiswould seem to be due to the low combustion tem-perature caused by the dilution of the fuel in thecombustion products.

The flame edge randomly moves in a horizon-tal plane, oscillating around a mean position. Anevaluation of the velocity of the displacement ofthe bright blue edge of the flame was providedby measurements made from video images. Theflame edge fluctuates aperiodically, with the max-imum edge velocity ranging up to about 0.6 m/s.

To determine the upstream conditions at theflame front, PIV and spontaneous emission tech-niques are coupled and a new moving coordinatesystem, attached to the flame edge in the PIVplane, is defined. A time average of the PIV fieldis calculated, allowing a filtering of the fluctuationof the flame. These fluctuations are attributed tolocal heterogeneities in the gas velocity (naturalconvection of air), temperature and compositionof the gas mixture ahead of the flame.

Increases of the fuel mass flux, _m00 between1.3 · 10�3 kg/m2/s and 2.4 · 10�3 kg/m2/s, leadto an increase in the size of the mean flame areaand a change in colour and shape. The flameenters a regime of cellular flaming, when the low-est parts of the flame are blue and the tips orange(radiation by the generated soot).

For higher mass fluxes, _m00 > 2:4� 10�3kg=m2=s, the flame becomes amber and orange andcovers the whole surface of the layer interface.

When the small soffit (Hsoffit = 0.20 m) is inplace and the floor is raised to retain a constantvent size, combustion can be sustained when thefuel flow rate is above 0.19 g/s, which equates to

–20 10 40 70mm 100 130

20

10

0–10

–20

–30

–40

–50

–60

–70

–80–90

mm–110

–100

–120

flame

Fig. 4. Example of the 2nd image from a pair from thePIV camera overlaid with the corresponding vector field;Hsoffit = 0.35 m, _m00 ¼ 1:2� 10�3kg=m2=s.

20

2530

3540

premixed flame diffusion flame

[mm]


_m00 ¼ 0:75� 10�3kg=m2=s. This means that thefuel flow rate needed to sustain combustion islower, but that a higher fuel flow rate relative tothe volume of the upper part of the compartmentis necessary. It can therefore be concluded that theheight of the space above the soffit plays a role,but that the link between the minimal fuel flowrate and the soffit height is non-linear. Tests withother soffit heights are necessary to determine thenature of the link.

With reference to Morehart et al. [7,8] andSugawa and Kawagoe [9], we conclude that thisflame, which resembles a light transparent blueveil flowing in the compartment detached fromany solid surface, can be called a ghosting flame.

A few tests were performed with methane andwith a heptane pool fire in which the same behav-iour was observed.

5.2. Temperature in the enclosure

Everywhere in the enclosure, two layers with aquasi-homogeneous gas temperature were mea-sured for a given _m00. The upper zone temperatureTupper is reported in Fig. 3 for a range of values of_m00. We observed a slight increase of Tupper with_m00. This phenomenon is attributed to a largerflame surface in the compartment, increasing theheat release inside the compartment and the heattransfer to the walls and ceiling (convectivemotion and radiation). The flame temperaturecannot accurately be determined due to theunstructured three-dimensional displacement ofthe flame.

5.3. Velocity measurements

Figure 4 shows an example of an image takenwith the PIV camera from the vertical medianplane of the compartment overlaid with the corre-sponding vector field. The image is the secondfrom a pair—the flame is only visible on the sec-ond due to the longer exposure time. PIV andspontaneous emission of the flame were coupledto link the instantaneous velocity field and theinstantaneous flame position. The statistical image

300

350

400

450

500

550

600

0 0,001 0,002 0,003 0,004

m" [kg/m^2/s]

T (

˚C)

Fig. 3. Temperature of the upper layer; Hsoffit = 0.35 m,thermocouple tree placed 0.1 m from rear wall, 0.14 mfrom right-hand wall; displayed values are averages fromheights 0.47 m, 0.59 and 0.69 m above the floor.

processing was done on 21 velocity fields (Fig. 5),the bright blue flame edge is at the origin; thethick line indicates the location of the flame.Though the spontaneous flame emission was spa-tially integrated, any ambiguity as to the locationof the flame edge in the PIV laser sheet could beresolved by observing the PIV vector field.

The gas enters the flame edge in a nearlyhorizontal trajectory from the rear of the compart-ment. Figure 6 reports the variation of the velocityvector Vedge at the flame structure (blue brightedge). A slight increase of Vedge is observed withthe mass flow rate of fuel varying between 0.26 g/s( _m00 ¼ 1:0� 10�3kg=m2=s, Vedge = 0.23 m/s)and 0.30 g/s ( _m00 ¼ 1:2� 10�3kg=m2=s, Vedge =0.35 m/s). Behind the edge, the velocity vectorsremain parallel to the sheet of flame (Fig. 5). Thevelocity and the angle of inclination both increase,achieving average values of 0.57 ± 0.05 m/s and22�, respectively. This flow acceleration can beattributed to thermal expansion and buoyancy

–40403020100–10–20–30–40

–35

–30

–25

–20

–15

–10

–5

0

510

15

[mm]

Fig. 5. ‘‘Average’’ velocity field calculated from 21 PIVmeasurements by performing a coordinate transferrelative to the edge of the flame. The location of theflame is indicated by the solid line; Hsoffit = 0.35 m,_m00 ¼ 1:2� 10�3kg=m2=s.

Fig. 6. Average velocity of the gas at the flame edge;Hsoffit = 0.35 m.

diluted fuel

premixedflow

air

rich premixed wing

diffusion flame

hot zone

lean premixed wing

burner

cold zone

fuel supply

recirculation of CP

Fig. 7. Schematic of the proposed combustion system.


effects. The reactants diffuse from each side of theflame, corresponding to a diffusion-type flame.

The buoyancy forces cause the sheet of flame toslope upwards and induce a recirculating structure(Fig. 7) with a characteristic length of the sameorder of magnitude as the soffit height Hsoffit. Thiseddy enhances the dilution of fuel with of combus-tion products and induces a convective motionwhich feeds the flame behind it. When the linesource burner is close to the rear of the compart-ment or to the entrance (distance less than the soffitheight), this recirculating motion becomes less sta-ble and the flame is more unstable for the same val-ue of _m00.

5.4. Chemical analysis

Gas samples were taken from a location aheadof and just slightly above the flame edge in therecirculative eddy. Analysis by gas chromatogra-phy shows that the mean gas composition in thisregion is:

• global fuel concentration 6.8% by volume;• combustion products CO2 8.8%, H2O 15%

(estimated from a carbon balance) and nitro-gen (69%);

• residual oxygen—2.8%—and minor specieswere also detected.

The data show that, a few centimetresupstream of the flame front but in the recirculativeeddy, a fuel rich mixture of fuel/oxygen/combus-tion products is formed. Air is mixed ahead ofthe flame edge.

5.5. Discussion

The average experimental flow velocity at theedge of the flame is 0.30 ± 0.05 m/s. This is closeto the laminar burning velocity SL of a premixedpropane flame (SL = 0.42 m/s at stoichiometryunder standard conditions); the difference mustbe attributed to the unknown dilution ratio ofthe reactants and the temperature ahead of theflame. These observations suggest that the stabili-

sation mechanism must be indeed of a triple flametype [12–14]. The bright edge of the flame is thelaminar premixed flame zone, which propagatestowards the rear of the compartment against theflow of the mixture of diluted fuel and air drivenby the natural convection in the enclosure. Theinclined tail in its wake, which shows characteris-tics of a diffusion flame, induces the recirculatorymotion. When _m00, the ratio of the mass flow rateof fuel _m and the area of the compartment Ac,increases above the extinction threshold level,the interface flame becomes more stable and sootproduction increases. This phenomenon can beinterpreted by an increase of the laminar burningvelocity due to the increase of the temperature(Fig. 3) and of the fuel available in the upper zone.

The recirculatory motion, induced by buoyan-cy forces, enhances the mixing of fuel with thecombustion products. The stability of a tripleflame increases with _m00, and decreases withincreasing Hsoffit, for wall to burner distances lessthan Hsoffit, and when the heat losses through thewalls in the upper zone increase; an increase ofHsoffit leads to larger scales of the recirculationeddy which decrease in stability.

6. Combustion regimes

The previous results clearly show that in thestudied cases the reaction zone is largely influ-enced by the fuel supply in the upper zone. Theairflow rate from natural convection is always suf-ficient to allow complete combustion of the fuel.

A new quantity is sought to characterise thecombustion regime at the interface: the averagearea covered by the flame inside the compartmentAf–i, and the area covered by the whole of theflame Af (determined by video image processing).It is assumed that the area covered by the flame isdirectly correlated with the released heat, andtherefore that the amount of heat released insidethe compartment, _Qi, can be studied by compar-ing the area of the part of the flame which is inside

Fig. 8. Average area covered by the flame in function ofthe fuel flux.


the compartment with that of the whole flame. Fig-ure 8 shows curves for the dimensionless averageflame sizes ðA�f�i ¼ Af�i=AcÞ and ðA�f ¼ Af=AcÞ asa function of the mass flux of fuel in the upper zone_m00, whereby Ac is the floor area of the compart-ment. The curves show that the flame size is wellcorrelated with _m00. For values of Af–i/Ac equal toone, the flame covers the whole surface of the com-partment. In this case, the premixed flame velocity,SL, at the blue bright edge is greater than the veloc-ity of the convective current upstream of the flame:the combustion is stable.

The break observed in the area variation at_m00 ¼ 1:9� 10�3kg=m2=s can be correlated to achange of the appearance of the flame (changein colour from blue to orange induced by soot for-mation). This result shows that the heat releaseinside the compartment is mainly related to _m00,though it can be noted that the air current fromnatural convection also depends on the fuel flow.In Fig. 8, the area of the flame A�f tends to zero forthe value of _m00 leading to a flame extinction. If weplot in Fig. 8 A�f with ( _m00 � _m00extinction), where_m00extinction is the minimum fuel flux needed to stabi-lise a flame, we obtain a linear relation as:

A�f ¼ 223 _m00 � _m00extinction

� �0:75 / _Q0:75i

This last relation confirms the correlation betweenthe total flame area and the heat dissipated in thecompartment, the difference between _Qi and _Q(rate of fuel supply) corresponding to the heatlosses (through the walls and by the vent) in theupper region.

In the studied range, the ventilation-controlledregime was not observed. To obtain such regimes,tests are underway with a restricted area at thecompartment aperture.

It can be concluded that the knowledge of theflame area seems to be a good criterion to charac-terise the combustion regime:

• Extinction for _m00 < 0:8� 10�3kg=m2=s()A�f�i ! 0;

• Instable interface flame for 0:8� 10�3

kg=m2=s < _m00 < 2:4� 10�3kg=m2=s() 0 <A�f�i < 1;

• Stable combustion for _m00 > 2:4� 10�3

kg=m2=s() A�f�i ¼ 1.

7. Conclusions

The behaviour of a fire fed by a fuel source (gasburner) in the upper layer of a two-layer configu-ration has been studied at laboratory scale inorder to enhance the knowledge of the flamestructure. Measurements of the spontaneous emis-sion from the flame, the temperature profile, gasconcentrations and the velocity field around theflame edge, were performed to identify the struc-ture and the stability domain of the reaction zone.

The results show that, under the studied condi-tions, the reactive region is located at the horizon-tal interface between the hot and clear air layers.Within the upper layer the buoyant forces inducea recirculatory velocity field with eddy size in theorder of magnitude of the soffit height. This leadsto the convection of diluted fuel towards theregion ahead the flame to stabilise it. The flamestructure has been interpreted as a triple flamewith a bright edge of premixed flame propagatingtowards the rear of the compartment against thedirection of the natural convective motion, andan inclined buoyant diffusion flame in its wake,which consumes the main part of the fuel. Thevelocity of the premixed flame relative to the airmotion determines the domains of extinctionand stable combustion, the flame velocity being,respectively, lower or higher than the convectivemotion. The characteristic parameters controllingthe combustion regime are identified as the con-sumption rate of fuel _m00 and the geometry ofthe compartment (size of the box and of the sof-fit). For the full open aperture, the fuel concentra-tion ahead the flame mainly controls thecombustion regime. The regime should changefor smaller values of the ventilation factor.

Within the range of parameter space studied,the surface area of the flame represents a good cri-terion to define the combustion regimes. Theflame stability increases with the injected massflow rate of fuel _m and decreases with increasingsoffit height (stability of the recirculation eddies)and with increasing surface of the compartment.

8. Outlook

Future experiments will also study the influ-ence of the nature of the fuel and of the ventila-tion factor Fv (vent flow) on the flame regimeand the compartment scale to obtain ventilation-controlled conditions.


Acknowledgments

This project is primarily financed by the Euro-pean Community’s Human Potential Programmeunder contract IHT-RTN-00-2, FIRENET. Theauthors thank the NFPA for their contributionto the work.

References

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Comments

Arnaud Trouve, University of Maryland, USA. Yourresults provide evidence of flow extinction phenomenaalong the stoichiometric interface that separates upperand lower layer gases. You emphasize the presence ofan edge flow that separates the burning and non-burningregions along the stoichiometric interface and proceed toexplain flame extinction as an edge flame propagationproblem. But flame extinction may be more readily ex-plained as a flammability limit problem for a pure air/vi-tiated fuel diffusion flame configuration. Could youplease comment on this alternative view point?

Reply. It is correct that the extinction of the flamewill be observed when it is no longer possible to achievea flammable mixture (pure air/vitiated fuel) at the inter-face. The concept of flammability limits certainly appliesfor premixed flames, but in the present case, we are look-ing at the establishment of a diffusion flame at the inter-face between the upper and lower layers. There is clearlyrecirculation within the upper layer, entraining air intothe interface region where flame will exist under the rightconditions. This will clearly depend on the degree towhich fuel in the upper layer has been vitiated. Estab-lishment of this diffusion flame at the limiting conditionrequires stabilization of the combustion process by thepremixed wings of a ‘‘triple flame’’ which must be ableto propagate against the flow at the interface. Clearly,

combustion can only be stabilized when the upstreammixture is within the relevant flammability limits.

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Carlos Fernandez-Pello, University of California

Berkeley, USA. It seems to me that your experimentalresults are strongly influenced by sedimentation on thegas in the ceiling layer. In that regard your conditionsare different than those in a fire where the gas is initiatedfor lack of oxygen but not necessarily by sedimentationof the gas.

Reply. Indeed, the combustion is strongly influencedby the supply of reactants. Several phenomena must beconsidered to answer your question. First, the diffusionof fuel from the top of the compartment to the interfaceis not sufficient to explain the supply of reactants to theflame: convective motion of fuel diluted by combustionproducts and air to the flame edge must be considered.Second, the flame is not extinguished by a lack of oxygenbut by too high a dilution of the fuel gas upstream of theflame edge. Finally, when methane is used as fuel (densi-ty lower than that of the combustion products or ofheated propane), the interface flame is always stabilised:the combustion cannot be supplied by gas layering, or‘‘sedimentation’’.

behaviour of a confined fire located in an unventilated zone

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